To implement a flexibly reconstructible network in the field of wavelength division multiplex communication, a wavelength-selective optical switch capable of inserting or branching an optical signal having an arbitrary wavelength into an arbitrary path is essential. The optical switch often uses a mirror device created by the MEMS (Micro Electro Mechanical Systems) technology as a means for deflecting light. This mirror device can be provided with a plurality of pivotal axes to make the mirror pivot. Hence, for example, in addition to a first pivotal axis that implements optical path switching, a second pivotal axis perpendicular to the first pivotal axis is provided. The mirror pivots about the second pivotal axis, thereby changing the optical loss. This allows to implement optical signal power control as well as optical path switching.
Such an optical switch can have two forms depending on its function: ADD type and DROP type. An ADD-type optical switch multiplexes optical signals with different wavelengths input from a plurality of input ports and outputs the multiplexed signal from at least one output port. On the other hand, a DROP-type optical switch demultiplexes a wavelength-multiplexed optical signal input from at least one input, port for each wavelength, and outputs the signals of the respective wavelengths from different output ports. These optical switch structures will be explained with reference to FIGS. 9 to 11.
An optical switch 100 shown in FIGS. 9 to 11 includes an input/output port array 110 having input and output ports (to be collectively referred to as “input/output ports” hereinafter) arrayed in a predetermined direction, a condenser optical system 120 formed from a first lens 121, a cylindrical lens 122, and a second lens 123, a diffraction grating 130, a third lens 140, and a mirror array 150 having a plurality of MEMS mirror devices 151 arrayed in a line along a predetermined direction. These components are arrayed in a line along the z-axis in the order named. The mirror of each MEMS mirror device 151 can pivot about the x-axis perpendicular to the z-axis and about the y-axis perpendicular to the x-axis. The input/output ports are arrayed in the direction along the y-axis. The MEMS mirror devices 151 are arrayed in the direction along the x-axis.
When the optical switch 100 is of the ADD type, the input/output port array 110 includes a plurality of input ports and one output port. In the ADD-type optical switch, signal light to be referred to as input light hereinafter) components input from the plurality of input ports are condensed on the diffraction grating 130 via the condenser optical system 120 and demultiplexed for each wavelength by the diffraction grating 130. Each of the demultiplexed signal light components of the respective wavelengths is condensed by the third lens 140 and enters a corresponding one of the MEMS mirror devices 151 of the mirror array 150. The signal light (to be referred to as output light hereinafter) components deflected by the MEMS mirror devices 151 are condensed by the third lens 140 and multiplexed by the diffraction grating 130. The thus wavelength-multiplexed output light is output from one output port via the condenser optical system 120.
On the other hand, when the optical switch 100 is of the DROP type, the input/output port array 110 includes one input port and a plurality of output ports. In the DROP-type optical switch, input light input from one input port is condensed on the diffraction grating 130 via the condenser optical system 120 and demultiplexed into the respective wavelengths by the diffraction grating 130. Each of the demultiplexed signal light components having the respective wavelengths is condensed by the third lens 140 and enters a corresponding one of the MEMS mirror devices 151 of the mirror array 150. Each of the output light components of the respective wavelengths deflected by the MEMS mirror devices 151 is output from a corresponding one of the output ports via the third lens 140, the diffraction grating 130, and the condenser optical system 120.
In the optical switch 100 having the above-described structure, making the mirror of each MEMS mirror device 151 of the mirror array 150 pivot about the x-axis indicates selecting an input port or an output port because the output light traveling direction changes to the array direction (y-direction) of the input/output port array 110. That is, an input port is selected in the ADD-type optical switch, and an output port is selected in the DROP-type optical switch. On the other hand, making the mirror of each MEMS mirror device 151 pivot about the y-axis indicates controlling the coupling ratio from the input port to the output port, that is, the light attenuation amount because the output light traveling direction changes to the direction (x-direction) perpendicular to the array direction of the input/output port array 110. The pivotal movement about the x-axis and that about the y-axis are combined. This allows the optical switch 100 to, upon input/output port switching, move the output light to make a detour around input/output ports other than the input output port of interest or give a predetermined light attenuation amount to optical coupling of the input/output port of interest.